Electrostatic saddle field collimating system



March 15, 1966 R. P. STONE 3,240,973

ELECTROSTATIC SADDLE FIELD COLLIMATING Filed March 1. 1962 2 Sheets-Sheet 1 INVENTOR. BYREJBERT 1? 5111115 ATTOKA/EX R. P. STONE 3,240,973

ELECTROSTATIC SADDLE FIELD COLLIMATING March 15, 1966 2 Sheets-Sheet 2 Filed March 1,

+10KM 25014 +701! 23 +451 i514 +90% INVENTOR. RUBERI F ST EINE drum/E) United States Patent 3,240,973 ELECTROSTATIC SADDLE FIELD COLLIMATING SYSTEM Robert I. 5tone, Lancaster, Pa., assignor to Radio Corporation of America, a corporation of Delaware Filed Mar. 1, 1962, Ser. No. 176,612 11 Claims. (Cl. 313-83) This invention relates to an electron beam system. In particular this invention relates to an improved electron beam collimating system. The invention is applicable to the collimation of either a focussed electron beam system or an unfocussed, flooding type electron system.

In the prior art there are certain tube types in which an extremely accurate collimating electron lens system is essential. In other words, in these tubes the paths of all of the electrons of a beam must be substantially parallel. In most cases the collimated beam is also parallel to the tube axis and thus perpendicular to a target electrode. One group of tubes which requires this extremely accurate collimating system is the group of tubes known as storage tubes. One storage tube type in which this problem is particularly pronounced is in the direct viewing or display storage tube. Therefore, this invention will be explained by way of example, in connection with a display storage tube.

In a conventional display storage tube there is an apertured insulating storage grid which is positioned closely adjacent to and between an output cathodolurninescent phosphor screen on one side and a collector mesh screen electrode on the other side. A writing electron gun is provided which deposits a charge pattern on the storage screen in accordance with incoming electrical signals applied to the writing gun. A viewing electron gun is also provided which directs electrons toward, or through, the storage screen to land on the phosphor screen in accordance with the signals written on the storage screen by the writing electron gun. Thus, a visible display of the charge pattern is produced on the output phosphor screen.

In tubes of the type briefly described above, it is extremely important that the viewing electron beam be accurately collimated so that extraneous output signals will not be generated and also so that actual signals are not obscured. As an example, it has been found that when using the prior art collimating systems, there may be variations in, for example, the thickness and position of electrically conductive wall coatings. Any such variations will adversely affect the entire beam collimating system of the tube. These mechanical variations must be minimized so that the electrons of the viewing beam will be properly collimated. Otherwise, these mechanical variations in tube structure will produce output signals in areas of the phosphor screen in which no incoming electrical signal has been stored on the storage grid, and/or the mechanical variations will obscure some signals which have been stored on the storage grid. The reason for this is that the stored signal may be only of the order of one or two volts magnitude. If the viewing beam does not approach the storage target in a well collimated fashion, its energy variation in the direction of approach may easily be larger than the one or two volts stored signal.

Certain mechanical variations in the prior art tubes could be eliminated by using restrictive manufacturing tolerance limits on the tube parts. Also, some of the poor collimation could be minimized by adjusting certain electrical collimating controls. However, these tubes are extremely expensive to manufacture when precise tolerances and controls must be maintained in order to 3,2lfifi73 Patented Mar. 15, 1966 secure a desired collimation. For example, the prior art tubes have a bulb wall coating in the conical portion of the bulb. In this structure, the conductivity, position and uniformity of the bulb wall coating must be accurately controlled as well as the dimensional tolerances of the envelope itself in order to decrease collimation errors.

It is, therefore, an object of this invention to provide an improved tube having a novel electron beam collimating system.

It is a further object of this invention to provide a novel collimating system for use in a storage tube characterized by its reduction of abberations and its decreased susceptibility to disturbances caused by mechanical variations in the component parts of the tube.

These and other objects are accomplished in accordance with this invention by providing an electron discharge device having a gradually graded voltage distribution at the boundaries of the electron lenses within the device, which makes an electron beam act as though it were in a system of substantially larger diameter than the actual physical size of the system. The boundaries of the electron lenses are also chosen to minimize disturbances caused by mechanical variations of the component parts.

The invention will be more clearly understood by reference to the accompanying single sheet of drawings wherein:

FIG. 1 is a schematic illustration of the equi-potential surfaces and the hypothetical viewing beam electron trajectories that occur in a tube of the prior art type ignoring space charge effects;

FIG. 2 is a schematic illustration of the equi-potential surfaces and hypothetical viewing beam electron trajectories that are produced in a tube utilizing a collimating system in accordance with this invention ignoring space charge eiiects; and,

FIG. 3 is a sectional view of a display storage tube utilizing this invention.

Referring now to FIG. 1, there is shown a group of equi-p0tential surfaces, together with calculated electron trajectories, from a viewing electron gun 30 to the plane of a collector electrode, represented by the solid line 13, that is positioned closely adjuacent to a storage grid. In the conical region of the tube, i.e. the conical portion between the neck and cylindrical portion of the bulb, there is a somewhat distorted saddle type field that is established by the potential applied to a bulb wall coating 12. This saddle field, in this region of the envelope, results in a strong bending of the electron beams as shown by the beam paths 11 in the conical region of the tube.

Since a large change in direction of the electron beam path occurs in the conical region of the tube, the collimating system that is presently used is critically dependent upon the bulb wall coating 12 in this area. Because of this critical dependency, any mechanical or electrical nonuniformities in the wall coating 12 will adversely affect the uniformity of the electron beam collimation.

If one plots the equi-potential lines for a full, nondistorted, saddle field lens, one finds a much more symmetrical potential distribution than that shown in FIG. 1. The reason for this difference is that the boundaries of the saddle field in the conical region of the bulb are confined by the gun 30, and by the smaller diameter of the conical bulb wall coating 12 so that these boundaries are substantially smaller than the rest of the lens system.

The abberations of a lens system for rays close to the axis are substantially smaller than those for rays near the outer edges of the lens, in other words, the electron paths near the axis are substantially less distorted than those near the wall coating 12. Therefore, if one could make a given beam throughout its physical extent appear to be in a field of twice the actual physical size of the lens, the abberations in the beam will be substantially reduced.

Therefore, a lens system is approximated in the arrangement of FIG. 2 embodying the invention, which system appears to the electron beam to be substantially larger than the actual physical size of the lens. Referring to FIG. 2, this lens system is approximated by a uni-potential collimating cylinder 37 in the central cylinder region of the tube. In addition, this apparent over sized lens system is approximated by regions of gradually varying potential between the ends of this collimating cylinder 37 and the adjacent radial end wall potentials. One region of gradually varying potential is achieved by means of a resistance spiral coating 32 in the conical portion of the envelope. The spiral resistive coating 32 extends from adjacent the end of the cylindrical collimating electrode 37 to the radial wall of the electron gun 30. At the other end of the collimating cylinder 37, the gradual potential variation is produced by a series of axially short collimating rings to which stepped potentials are applied. It should be understood that although two collimating rings 36 and 38 have been shown, one or more may be used. The larger the number of collimating rings that are used, the more gradual will be the stepped potential variation. Thus, the saddle field has been moved to a region substantially within a cylindrical electrode, where it is established by an element which is easily subject to precise mechanical tolerances and the voltage adjacent to both ends of the cylindrical electrode is a gradually varying function.

The radial walls of the saddle field lens illustrated in FIG. 2 are represented by the plane of the collector grid, schematically represented as a line 13, on one end and the plane of the bulb spacer support attached to the viewing gun 30 on the other end. The collimating cylinder 37 is electrically isolated from the collector grid by a series of collimating rings 36 and 38 at intermediate potentials, and from the electron gun 30 at the other end by the resistive spiral 32. Equi-potential lines, measured on a resistance network analog, together with calculated electron trajectories are shown. It should be noted that in the region enclosed by the conical portions of the bulb, where the spiral resistive coating 32 has its greatest effeet, the bending of the electron beams L1 is quite small. The saddle field region of strong beam bending has been shifted to the region enclosed by the cylindrical portion of the envelope, i.e. region where it is established by the cylindrical electrode element 37 which may be accurately and economically manufactured and controlled. Therefore, irregularities of the bulb wall and its coating have a minimum effect on the uniformity of the collimation obtained in the tube. Since the cylindrical electrode 37 can be accurately machined, to rather close tolerances at a low cost, the collimating system of this invention is not only more accurate but can also be produced economically.

Referring now to FIG. 3, there is shown an embodiment of this invention comprising a direct viewing storage tube 18. The tube 18 comprises an evacuated envelope 20 having a neck portion 19, a conical portion 21 extending from the neck portion 19 and a cylindrical portion 23 joined to the conical portion 21. At one end of the tube 20 there is positioned a cathodolurninescent phosphor screen 22. Adjacent to the phosphor screen 22 is an apertured storage grid 24. Closely adjacent to the apertured storage grid 24 is a collector grid 26. Positioned in the other end of the envelope 20 is a writing electron gun 28 and a viewing electron gun 30. Both the writing and viewing electron guns are schematically shown for simplicity of illustration. Extending along the conical portion or wall 21 of the envelope 20 is a long continuous strip of resistive material in the form of a spiral coating 32. The spiral resistive coating 32 has one end connected to a potential at, or near, that of the second accelerating electrode 34 of the viewing gun 30, and extends along the conical portion 21 of the bulb 20 adjacent to or beyond one end of the collimating cylinder 37 and terminating at a voltage at, or near that of the collimating cylinder 37. The spiral resistive coating may have approximately 16 turns per inch with a spacing of approximately .030" between turns and may be made of a material such as aquadag, applied as a carbon suspension. It should be understood that the continuous resistive spiral 32 may be economically manufactured since any misposition, or varying thickness therein will not substantially affect the gradually varying voltage produced by such a continuous resistive spiral.

Adjacent to the other end of the collimating cylinder 37, and supported by an end thereof, as shown, is a first collimating ring 36. Supported by an end of the first collimating ring 36 is a second collimating ring 38. Supported on an end of the second collimating ring 38 is a collector electrode ring 40 which in turn supports the collector grid 26. The storage grid 24 is supported across a storage grid ring 42 which is supported in turn from the collector grid ring 40. The various cylinders or rings 37 through 42 are electrically insulated but are mechanically connected for support reasons. One method of forming such a support structure may be found in a copending application of Raymond G. Spangler, Serial No. 90,795, filed on February 21, 1961.

Generally, the direct viewing storage tube operates, with potentials such as those shown in FIG. 3 as an example, in such a manner that writing gun 28 provides a focussed beam of electrons to produce a charge pattern on the storage grid 24. The viewing gun 30 produces a spray of electrons which passes through the storage grid 24 to land on the phosphor screen 22 in accordance with the charge pattern established by the writing gun 28. Greater detail of the operation and structure of a direct viewing storage tube may be found in US. patent to Knoll No. 2,856,559.

The electrons from the viewing gun 30 are accurately collimated before they pass through the collector grid 26. This accurate collimation is provided by establishing fields, or gradients, which effect the beam as though two fields were set up by elements which are larger than the actual physical size of the envelope. The effectively larger fields are established by shifting the saddle lens to a position in the envelope where it is not distorted; by shifting the saddle field lens to a position in the envelope where it may be accurately and economically manufactured; by establishing gradually varying voltage gradients adjacent to both ends of the saddle field; and by providing a means for establishing the gradually varying voltage gradients which may be accurately controlled while still economically manufactured.

What is claimed is:

1. A direct viewing storage tube comprising an envelope having a conical portion, means for producing a saddle field in said envelope in an area removed from said conical portion, means for gradually decreasing the voltage in the region of said tube extending from adjacent the smaller end of said conical portion to adjacent one boundary of said means for producing said saddle field and means for gradually increasing the voltage in the region of said tube extending from adjacent the other end of said means for producing said saddle field.

2. A display storage tube comprising an envelope, an electron gun positioned in One end of said envelope, a storage grid positioned in the other end of said envelope, a collimating electrode positioned in said envelope and between said electron gun and said storage screen, at, least one collimating ring positioned between said colli-. mating electrode and said storage grid, and means between said collimating electrode and said electron gun for gradually decreasing the voltage in the region of said; envelope extending from said electron gun to said c l l i mating electrode.

3. A display storage tube comprising an envelope, an electron gun positioned in one end of said envelope, a storage grid positioned in the other end of said envelope, a collimating electrode positioned in said envelope and between said electron gun and said storage grid means for producing a decreasing voltage gradient in said envelope from said storage grid to said collimating electrode, and a continuous spiral resistive coating extending from adjacent said electron gun to adjacent said collimating electrode and on the inner surface of said envelope for producing a decreasing voltage gradient in said envelope from said electron gun to said collimating electrode.

4. A display storage tube comprising an envelope, and electron gun positioned in one end of said envelope, a storage grid positioned in the other end of said envelope, a collimating electrode positioned in said envelope and between said electron gun and said storage screen, a continuous spiral resistive coating extending from adjacent said electron gun to adjacent said collimating electrode on the inner surface of said envelope, and at least one collimating ring positioned between said collimating electrode and said storage screen and Within said envelope, said spiral resistive coating and said collimating ring producing increasing voltage gradients extending from the ends of said collimating electrode.

5. A direct viewing storage tube comprising an envelope, an electron gun, a storage screen positioned Within said envelope, means for producing an electrostatic saddle field within said envelope, means for gradually increasing the voltage from adjacent one end of said means for producing said saddle field to adjacent to said electron gun, and means for gradually increasing the voltage from adjacent the other end of said means for producing said saddle field to adjacent said storage screen.

6. A storage tube comprising an envelope, an electron gun positioned substantially on the axis of said envelope, means for producing an electrostatic saddle field in said envelope in a region removed from said electron gun, a continuous spiral coating extending from adjacent said electron gun to adjacent one end of said means for pro ducing said saddle field, and at least one collimating ring positioned between the other end of said saddle field and the end of said envelope remote from said electron gun, said spiral coating and said collimating ring producing gradually increasing voltage gradients extending from two opposite terminal regions of said saddle field producing means.

7. An electron beam tube comprising an envelope, an electron gun in one end of said envelope for producing an electron beam, a target electrode in the other end of said envelope and in the path of said electron beam, means for producing an electrostatic saddle field in said envelope through which said electron beam passes in its transit from said electron gun to said target electrode, means including a spiral resistive coating on inner wall of said envelope for gradually increasing the voltage in the region of said tube from adjacent the ends of said means for producing said saddle field, to adjacent to said target electrode and said electron gun.

8. A storage tube comprising an envelope having a substantially conical portion and a substantially cylindrical portion an electron gun in said envelope, and collimating electrode means for establishing an electrostatic saddle field in said substantially cylindrical portion of said envelope, said means comprising a collimating electrode in said cylindrical portion, and means in said conical portion and in said cylinder portion adjacent to opposite ends of said collimating electrode for producing gradually increasing voltage gradients extending from said ends.

9. A direct viewing storage tube comprising an envelope, at least one electron gun for producing an electron beam in said envelope, a storage target in said envelope and in the path of said electron beam, and means surrounding said electron beam path for collimating said electron beam in its transit from said electron gun to said storage target, said means including a tubular collimating electrode, and said means further including a continuous spiral resistive coating on the inner surface of said envelope, one end of said spiral resistive coating being positioned adjacent to one end of said tubular collimating electrode, and a ring electrode positioned adjacent to the other end of said collimating electrode.

10. A storage tube comprising;

(a) an elongated envelope,

(b) an electron gun for producing an electron beam in one end of said envelope,

(c) an apertured storage member in the other end of said envelope and in the path of said electron beam,

(d) means for collimating said electron beam during its transit from said electron gun to said apertured storage member.

(c) said means including a substantially tubular electrode for producing an electrostatic saddle field, and

(f) means for gradually decreasing the voltage in the regions of said tube extending from said electron gun and said storage member to adjacent the ends of said substantially tubular electrode.

11. A storage tube comprising:

(a) an elongated envelope,

(b) an electron gun for producing an electron beam in one end of said envelope,

(c) an apertured storage member in the other end of said envelope and in the path of said electron beam,

(d) means for collimating said electron beam during its transit from said electron gun to said apertured storage member,

(e) said means including a substantially tubular electrode spaced around said electron beam path for producing an electrostatic saddle field,

(f) at least two collimating ring electrodes spaced around said electron beam path and between said substantially tubular electrode and said apertured storage member, and

(g) a continuous resistive spiral electrode on the inner Wall of said envelope and around said electron beam path and between said electron gun and said substantially tubular electrode,

(h) said collimating ring and said spiral electrode pro ducing voltage gradients increasing from the ends of said tubular electrode and towards said electron gun and said storage member.

References Cited by the Examiner UNITED STATES PATENTS 2,141,414 12/1938 Schlesinger 313-83 2,209,159 7/1940 Gorlick et al. 31516 3,005,927 10/1961 Godfrey 31381 X 3,040,205 6/1962 Walker 31381 X FOREIGN PATENTS 615,563 1/ 1949 Great Britain.

666,382 2/ 1952 Great Britain.

735,463 8/ 1955 Great Britain.

GEORGE N. WESTBY, Primary Examiner.

ARTHUR GAUSS, Examiner. 

8. A STORAGE TUBE COMPRISING AN ENVELOPE HAVING A SUBSTANTIALLY CONICAL PORTION AND A SUBSTANTIALLY CYLINDRICAL PORTION AN ELECTRON GUN IN SAID ENVELOPE, AND COLLIMATING ELECTRODE MEANS FOR ESTABLISHING AN ELECTROSTATIC SADDLE FIELD IN SAID SUBSTANTIALLY CYLINDRICAL PORTION OF SAID ENVELOPE, SAID MEANS COMPRISING A COLLIMATING ELECTRODE IN SAID CYLINDRICAL PORTION, AND MEANS IN SAID CONICAL PORTION AND IN SAID CYLINDER PORTION ADJACENT TO OPPOSITE ENDS OF SAID COLLIMATING ELECTRODE FOR PRODUCING GRADUALLY INCREASING VOLTAGE GRADIENTS EXTENDING FROM SAID ENDS. 